Preparation of reducing agents



Patented Apr. 25, 1944 PREPARATION OF REDUCING AGENTS Joseph Frederic Walker, Lewiston, N. Y., assignor to E. I. du Pont de Nemours & Company, Wil mingtcn, Del., a corporation of Delaware No Drawing. Application February 21, 1942, Serial No. 431,817

4 Claims.

This invention relates to the manufacture of formamidine sulfinic acid, also known as iminoamino-methane sulfinic acid. The invention has for its object the provision of an economical and eificient process for manufacturing this compound on a commercial scale from the inorganic starting compound, ammonium thiocyanate. A further object of the invention is to provide a method of producing formamidine sulfinic acid in good yield and high quality, by simple steps of procedure which may be readily adapted to operations on a large scale.

Formamidine sulfinic acid, a compound be lieved to have a structure corresponding to the following formula HN O ' HiN is ordinarily prepared by oxidizing thiourea with hydrogen peroxide. It is useful for various purposes, among which is its employment as a reducing agent for vat dyes.

There are references in the chemical literature, particularly the methods of Barnett and of Vanino and Schinner, to procedures by which this compound can be prepared. Both methods involve the use of solid thiourea. However, :both of the procedures have been found deficient for commercial operations, that of Barnett giving only a low yield of product, and that of Vanino and Schinner leading to an impure product. There are further disadvantages in the methods described in the chemical literature, particularly the violence of the reactions; the bulky character of the process, requiring the handling of large volumes of reaction mixture; and, in the case of Vanino and Schinner, the high degree of impurity of the product.

An improved method of operation is described in the Havas Patent 2,150,921, whereinthe thiourea and hydrogen peroxide are both utilized in the form of their solutions. By the use of solutions, by carefully controlling the reaction to prevent overheating, and by observing other precautions, it is possible by this process to secure the desired product in satisfactory yields and high purity, the procedure readily lending itself to large-scale commercial operations. However, the method of Havas, as the earlier methods described in the chemical literature, utilizes the organic compound, thiourea, as the starting material, and has the disadvantage that it requires relatively pure thiourea.

ammonium thiocyanate.

I have now developed an improved process by which formamidine sulfinic acid may be produced in high yields from mixtures of ammonium thiocyanate and thiourea. My improved process for producing formamidine sulfinic acid preferably utilizes the equilibrium mixture containing ammonium thiocyanate and thiourea, prepared by heating ammonium thiocyanate at a temperature sumciently high to fuse the mass, followed by cooling the resulting product, although any mixture of ammonium thiocyanate and thiourea is utilizable in my process. In this Way it is possible to produce the desired product from starting materials of an inorganic character, ammonium thiocyanate and hydrogen peroxide, thus avoiding the necessity present in earlier processes for starting with substantially pure thiourea.

By carefully regulating the temperaturev at which the reaction betweenthe hydrogen peroxide and a mixture of ammonium thiocyanate and thiourea is carried out, substantially all of the hydrogen peroxide will preferentially react with the thiourea, although at higher temperatures reaction between the hydrogen peroxide and ammonium thiocyanate would occur. This improved process, readily adaptable for production of high purity product on the commercial scale, utilizing as the starting materials hydrogen peroxide and a mixture of ammonium thiocyanate and thiourea, is set forth in greater detail below. While any mixture of ammonium thiocyanate and thiourea may be reacted with the hydrogen peroxide to produce formamidine sulfinic acid, the temperature being carefully controlled so as to prevent any substantial reaction between the hydrogen peroxide and thiocyanate, ordinarily the mixture utilized is one prepared by heating When preparing this solid equilibrium mixture of ammonium thiocyanate and thiourea the ammonium thiocyanate is first melted; by suitable heating, followed by permitting the resulting equilibrium mixture to solidify. For example, the ammonium thiocyanate may be heated for one hour at a temperature of 170 C. and then solidified by cooling.

The solid equilibrium mixture is preferably reacted with hydrogen peroxide by bringing it into contact with relatively concentrated hydrogen peroxide, for example the commercial volume solution (27.6% H202 by weight). In order to prevent reaction between the thiocyanate component and the hydrogen peroxide in any substantial amount, it is essential carefully to con- 'trol the temperature rise resulting from the exothermic character of the reaction between peroxide and thiourea. This is most effectively accomplished by cooling the reaction mixture, either with ice, or by means of air or liquid cooling. I have found that the temperature must not exceed about 30 C. if the reaction between thiourea and hydrogen peroxide is to proceed with the formation of the desired product in a high state of purity and in satisfactory commercial yields. At temperatures above 30 C., such as a temperature of 40 C., the resulting product is not only impure, being contaminated with a yellow amorphous by-product, but the amounts of formamidine sulfinic acid crystallizing out on cooling are relatively small.

It is therefore necessary to prevent the temperature during the reaction from going above 30 C., by providing the required amount of cooling. In recovering the product, it is desirable to cool the reaction mixture to about C., in order that the product may crystallize out. Generally I have found that the yields are better when the entire process is carried out so that the temperature at no time exceeds 10 C. and, while it is possible to operate and secur satisfactory yields at temperatures up to 30 0., my preferred procedure contemplates carrying out the process at a temperature which does not exceed about 10 C. In any event, at the conclusion of the reaction, the reaction mixture should be cooled to about 10 C. or below in order that a large portion of the product may crystallize out, permitting its ready recovery.

It has been found that the yield of formamidine sulfinic acid is substantially quantitative, based on the amount of hydrogen peroxide utilized, when the reaction mixture is maintained at 10 C. or below. However, only slightly more than half of the product will crystallize out, when the salt concentration is 17% by weight, the remainder being dissolved in the reaction mixture. All the thiourea is not converted to formamidi'ne sulfinic acid, but yields based on this component are 80% or better. Addition of excess hydrogen peroxide is undesirable, and the amount added should be based on the thiourea content of the thiocyanate-thiourea melt. Excess peroxide if present will react with thiooyanate and dissolved product when the solution is warmed after filtering off precipitated product, and is thus wasted. Unreacted ammonium thiocyanate and thiourea can be recovered and reutilized.

As the temperature maintained during the reaction is increased, the yield drops to about 82% at 20 C. and to 79% at 30 0., both based on the hydrogen peroxide utilized, the amount of solid product crystallizing out in each case upon cooling to 10 C. also decreasing to some extent. However, operation at these temperatures is satisfactory, giving yields which are regarded as very high for commercial operation. At temperatures above 30 C. the yield may be reduced to 60% or below and the product is contaminated with a yellow by-product resulting, apparently, from reaction, substantial in amount at this temperature, between the ammonium thiocyanate present and the hydrogen peroxide. When temperatures as high as 50 C. are permitted during the reaction, I have found that substantially no solid product is crystallized out on cooling to temperatures of 10 C. or below.

Without limiting my invention to any particular procedure, the following example will illustrate a preferred mode of operation. All parts specified therein are parts by weight.

Example A melt containing ammonium thiocyanate and thiourea in equilibrium amounts was prepared by melting solid ammonium thiocyanate, the heating being carried out at 170 C. for approximately one hour. Upon solidification a solid mixture of ammonium thiocyanate and thiourea resulted.

Water in the amount of 750 parts was used to dissolve 150 parts of the equilibrium melt. The resulting solution was divided into five aliquot portions, each portion containing 30 parts of the equilibrium mixture dissolved in 150 parts of water. The thiourea present in each of these portions was reacted with volume hydrogen peroxide solution (27.6% H202 by weight) under the following temperature conditions:

1. The first aliquot portion was reacted with 26 parts of 100 volume hydrogen peroxide solution (equivalent to 2.5 parts H202), the temperature being carefully controlled during the reaction so' that it did not exceed 10 C. Upon cooling, 3.9 parts of substantially pure formamidine sulfinic acid crystallized out. This amounted to approximately 52% of the yield, which was practically quantitative based on the amount of thiourea reacted. The remainder stayed in solution, from which it could be recovered by any convenient process.

2. The second aliquot portion was also reacted with 26 parts of 100 volume hydrogen peroxide solution, the temperature being controlled during the reaction so that it did not exceed 20 C. Upon cooling to 10 C., 2.5 parts of the desired product crystallized out. A larger amount of the formamidine sulfinic acid remained in solution, the yield, based on thiourea reacted, being approximately 82%.

3. The third aliquot portion was reacted with 26 parts of 100 volume hydrogen peroxide solution, the temperature during the reaction being maintained at 30 C. or below. Upon cooling to 10 C., 2.3 parts of the product crystallized out, the remainder remaining in solution. The yield in this case was approximately 79% based on thiourea.

4. The fourth aliquot portion was reacted with 26 parts of 100 volume hydrogen peroxide solution, the temperature being carefully controlled so that it did not exceed 40 C. Upon cooling to 10 C., only 0.9 part of a highly impure product, contaminated with a yellow amorphous byproduct, was secured. The yield of this impure product was also low, being 60% or less.

5. The fifth aliquot portion was reacted with 26 parts of 100 volume hydrogen peroxide solution, the temperature not being allowed to exceed 50 C. Upon cooling to 10 C. none of the desired product was secured, although some possibly may have been present in the reaction mixture. However, there were no indications that the product was formed in anysubstantial yield.

The rate of feeding the hydrogen peroxide solution to the solid equilibrium melt of ammonium thiocyanate and thiourea, or to the solution of the melt, is governed by the temperature developed during the reaction. The reaction being exothermic, cooling is generally necessary to keep the reaction temperature at 30 C. or below and the temperature may likewise be controlled to some extent by the rate of addition. While I have indicated the desirability of using the commercial 100 volume hydrogen peroxide solution, it is by no means necessary to utilize hydrogen peroxide of this concentration, as solutions of lower or higher concentration,-such as those used by Barnett or by Vanino and his co-worker, may be employed.

In preparing the thiocyanate-thiourea equilibrium melt, the temperatures and times suggested are merely illustrative. It may be prepared by any procedure which involves melting and cooling the ammonium thiocyanate, such as by heating at 180 C. for forty-five minutes and thereupon permitting the product to solidify by cooling. It is, of course, not necessary to employ this equilibrium melt, as any mixture of ammonium thiocyanate and thiourea may be utilized in my process, the temperature of the reaction mixture being carefully controlled so that no substantial reaction between the thiocyanate and hydrogen peroxide occurs. I have found the commercial ammonium thiocyanate, such as the so-called Koppers solid commercial product, entirely satisfactory as a starting material.

The details given in the above example are merely illustrative and the procedure may be varied within wide limits as to quantities employed, and other details, within the skill of those engaged in the art.

It will be apparent from the above that I have developed an improved and efficient method for producing imino-amino-methane sulfinic acid from inorganic starting materials by the use of a procedure wherein reaction between the thiourea and hydrogen peroxide is favored, the conditions being controlled so that the hydrogen peroxide does not react with the ammonium thiocyanate component in any substantial amount. The process is one producing the desired product in high yield and in pure form, and it is readily adapted for production of the desired product in large-scale operations.

I claim:

1. The process for preparing formamidine sulfinic acid which comprises reacting an aqueous solution of hydrogen peroxide and an aqueous solution of an equilibrium melt comprising thiourea and ammonium thiocyanate, the temperature of the reaction mixture being controlled durme said reaction so that it does not exceed about 30 C.

2. The process for preparing formamidine sulfinic acid which comprises reacting, in aqueous reaction mixture, hydrogen peroxide and an equilibrium melt comprising ammonium thiocyanate and thiourea, said equilibrium melt having been prepared by melting and solidifying ammonium thiocyanate, the temperature at which the reaction is carried out being controlled so that it does not exceed about 30 C.

3. The process for producing formamidine sulfinic acid which comprises reacting, in aqueous reaction mixture, hydrogen peroxide and an equilibrium melt comprising ammonium thiocyanate and thiourea, the temperature of the reaction mixture being controlled during said reaction so that it does not exceed about 10 C.

4. The process for preparing formamidine sulfinic acid which comprises reacting, in aqueous reaction mixture, hydrogen peroxide and an equilibrium melt comp-rising ammonium thiocyanate and thiourea, the temperature of the reaction mixture being controlled so that it does not exceed about 30 C., and then cooling said reaction mixture to a temperature not exceeding 10 C. in order to crystallize out formamidine sulfinic acid.

JOSEPH FREDERIC WALKER. 

